The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. Combustion within an engine involves introducing fuel and air into a combustion chamber and causing the ignition of the fuel air mixture or charge to harness the work created thereby. Introduction of the fuel and air and the ignition are timed by the controlling feature of the combustion chamber, the position of the piston acting within the cylinder, modulating the volume and therefore the ultimate conditions within the combustion chamber. Timing of the events involved in combustion and the properties of the resulting combustion affect the resulting efficiency and emissions of the engine.
Combustion timing or phasing is useful to diagnose issues in the combustion process. The magnitude of the resulting pressure or the action upon the piston is also useful to diagnose these issues. For a normal combustion process operated under a particular set of parameters, combustion results are predictable to within a small range. Combustion cycles deviating from this small range indicate that conditions within the combustion chamber are outside of the expected parameters. Analysis of combustion cycles may be performed in a number of ways.
Known methods to evaluate magnitude measured as signal power or to evaluate combustion phasing rely on estimating heat of combustion, the work performed by combustion, or other reactive metrics. These methods review historical data and react to trends or accumulated data points in the combustion data. However, compression-ignition engines and other engine control schemes operate over broad engine conditions. Effective and timely control, including fuel control, fuel tailoring, charge ignition timing control, exhaust gas recirculation (EGR) control, is necessary to meet operator demands for performance and fuel economy and comply with emissions requirements. Furthermore, there is much variability, including that related to: components, e.g., fuel injectors; systems, e.g., fuel line and pressures; operating conditions, e.g., ambient pressures and temperatures; and fuels, e.g., cetane number and alcohol content. The variability in combustion affects heat release and work output from individual cylinders, resulting in non-optimal performance of the engine. A measure of combustion variability based on real-time engine performance would be valuable to diagnose instability in the combustion process and provide information useful to reduce periods of inefficient or high emission operation.
Methods are known for calculating combustion metrics, in the form of either magnitude or combustion phasing, based upon direct measures of the results of combustion, such as direct pressure measurements from the combustion chamber or the net crankshaft speed resulting from combustion. Additionally, methods are known for processing complex or noisy signals and reducing them to useful information. One such method includes spectrum analysis through Fast Fourier Transforms (FFT). FFTs reduce a periodic or repeating signal into a sum of harmonic signals useful to transform the signal into the components of its frequency spectrum. Once the components of the signal have been identified, they may be analyzed and information may be taken from the signal. Such evaluations may be especially important in engines operating under homogeneous charge compression ignition (HCCI), compression ignition such as is implemented in diesel applications, or other auto-ignition schemes, as small variations in cylinder conditions can interfere with conditions necessary to create efficient and orderly auto-ignition necessary to derive the benefits of efficiency, fuel economy, and low emissions evident in a properly functioning engine.
However, pressure sensors in particular are prone to failure. Pressure sensors in direct communication with the combustion chamber are subject to rapid and extreme changes in pressure. Changes to engine settings in response to perceived combustion issues diagnosed by pressure sensors can have a drastic effect on engine performance, especially if the underlying pressure readings are unreliable.